Ancient giants unearthed in South Africa
Dr Adam Yates
The archetypal dinosaur image that most of us hold is very much that of a
sauropod - the classic, tiny-headed, long-necked, elephantine-limbed giant.
Despite this there are vast gaps in our knowledge about the sauropod group.
Fortunately a series of discoveries in South Africa is starting to fill in some
vital missing pieces.
A bed of bones belonging to a type of sauropod called Vulcanodon, was
recently identified in South Africa and an excavation is currently underway.
Just as tantalising was the discovery of a truly ancient sauropod in the famous
Karoo Basin of South Africa, near the town of Ladybrand. Named Antetonitrus
in 2003, this sauropod is 215 million years old (Late Triassic Period), which is
close to the beginning of the history of this group.
What is already known about sauropods? The group includes the largest of all
dinosaurs, indeed the largest of all terrestrial animals ever to have existed.
The very biggest may have reached 40 metres in length and weighed close to 100
metric tons.
Sauropods were ecologically significant and dominated the large terrestrial
herbivore niches in most parts of the world from 170 million years ago (Middle
Jurassic) to the end of the dinosaur age - 65 million years ago (Late
Cretaceous).
The closest relatives of the sauropods, a group of primitive dinosaurs known
as prosauropods, extends back to the Late Triassic (about 220 million years ago)
implying that the sauropods may also have had a history extending back as far.
Thanks largely to a series of spectacular discoveries in South America
palaeontologists are now confident that the common ancestor of all dinosaurs was
a small to medium sized (no more than 2 metres long and weighing less than 50
kg), carnivorous biped.
Sauropods were very much transformed from this common ancestor. They were
strictly herbivorous and had wide semicircular, or rectangular cropping jaws
packed with spoon-shaped teeth for taking big mouthfuls of fodder. Unlike the
teeth of most reptiles, including those of the ancestral dinosaurs, the teeth of
sauropods contact the teeth in the opposite jaw in precise occlusion. Sauropod
teeth have prominent wear-facets indicating that they chewed their food to a
substantial degree. Other features of the sauropod skull that are unique include
their greatly enlarged nostrils, which are retracted away from the snout tip so
that they occupy a place high on the skull roof. The biological significance of
this change is not well understood.
The sauropod neck is extremely elongated and includes at least two more
vertebrae than its ancestors, with a total of at least 12. One genus of sauropod
has a staggering 19 vertebrae in the neck, the largest number found in any
vertebrate. Clearly sauropods had a long reach and thus had a very large
'feeding envelope' that they could exploit without having to walk a step.
The bipedal nature of the ancestral dinosaur indicates that sauropods were
only secondarily quadrupedal animals, which dropped back onto all fours early in
their evolution. This meant that their grasping hand had to be modified for
walking upon. This required two significant changes. Firstly the hand had to be
rotated so that it swung in a fore-aft plane (we call such hands 'pronated') as
opposed to the transverse plane of bipedal dinosaurs. To achieve this, the bones
of the forelimb (radius and ulna) twisted their positions relative to one
another, and their shape at the wrist joint is such that they were locked into
position, preventing rotation of the hand. Secondly the hand had to be modified
to bear the huge weight of a sauropod body. To achieve this the palm of the hand
was modified into a semi-tubular colonnade of equal-length bones. This colonnade
bore all of the weight, consequently the fingers were not needed and faded away.
Other limb modifications include straightened column-like posture, reduced
muscle attachments and broad, spreading hind feet. All these indicate that
sauropods held their limbs straight under their body and moved slowly, never
taking big steps and never taking more than two feet off the ground at any one
time.
Clearly sauropods present us with many questions on how such prodigious
animals functioned as living organisms and how such dramatic evolutionary
changes occurred. Unfortunately the earliest sauropod fossils are substantially
incomplete, telling us little about the early members of the group. The most
informative of these early sauropod fossils is Vulcanodon, from the
shores of Lake Kariba in Zimbabwe. This fossil is 180 million years old, which
places it in the later part of the Early Jurassic.
The most informative parts of Vulcanodon are its limbs. (The skull,
neck, trunk vertebrae and hands are all missing.) Although the limbs are
substantially more primitive than later sauropods, they are already highly
specialised and show that the animal was a large, fully quadrupedal animal with
columnar limbs and a short foot, indicating that it had adopted the slow,
stiff-legged gait of later sauropods. The radius and ulna of the forelimb are
twisted indicating that the hand was pronated, but without the hand bones we
cannot tell if it was modified into the tubular, fingerless structure of later
sauropods.
The Karoo Basin's Antetonitrus skeleton (mentioned in the first
paragraph) is by no means complete, but there are bones from all major regions
of the skeleton. These show a small, heavily-built sauropod (perhaps 10 metres
long and weighing 1.5 tons) caught in the evolutionary transition from a
prosauropod-like body to a typical sauropod. Most intriguingly the evidence
shows that although Antetonitrus had evolved full quadrupedalism (for
instance its forelimb was nearly as long as its hind limb), modification of the
hand lagged behind and it remained a primitive grasping organ. Unfortunately
none of the skull and very little of the neck was found so we cannot yet know
what changes had occurred in these regions.
Nevertheless the Karoo is far from exhausted and preliminary investigations
show that there are many significant fossils still to be excavated, including
more bones at the Antetonitrus site. Analysis of some of the Karoo's
previously named dinosaurs, for example Melanorosaurus, suggests that
they too are primitive sauropods, rather than prosauropods as previously
identified. The Karoo may be viewed as the 'cradle of sauropods' and we can look
forward to many questions about the origin of these dinosaurian titans being
answered by palaeontologists in South Africa.
More information:
Dr Adam Yates Email: YatesA@geosciences.wits.ac.za
Dr Yates will be delivering a lecture at the Sasol Scifest 2004: "South
Africa and the origins of the dinosaur titans". More information on the
Science festival and Dr Yates talk at: www.scifest.org.za
|
